A highly efficient and safer method of bulk mining entitled Vertical Retreat Mining ("VRM") has been recently developed to more expeditiously mine ore from underground mines. In brief, in order to continue to work an underground excavation, large diameter blast holes are drilled into the floor of an upper stope (or drift) vertically disposed above a lower stope. The stopes may be separated by up to 750 feet (228.6 meters). These blast holes of various depths are drilled in predetermined patterns and filled with the appropriate explosive in order to loosen the ore and rock disposed between the two levels for subsequent removal and treatment.
Currently, there are a number of in-the-hole ("ITH") drills that accomplish the above task. However, they have a number of drawbacks that oftentimes impede efficient drilling. Current machines require about 121/2 feet (3.8 m) of back height to drill a vertical hole with a standard 5 foot (1.5 m) drill rod. The drive head is usually vertically driven by a long double ended hydraulic cylinder; a ganged triple hydraulic cylinder arrangement with one central cylinder flanked by two side cylinders; or a cumbersome chain and sprocket arrangement. The masts on such machines require so much height that it is impossible to drill in low back height areas. Accordingly, even when such was unnecessary a stope would have to be enlarged to at least twelve and a half feet high just to accomodate the ITH drill.
Similarly, these machines, due to their size, and physical construction, could not easily be transported through small openings. Rather than spending time, money and manpower on the productive business of mining and recovering ore, valuable time and money is wasted simply in making areas large enough to accomodate these drills. Moreover, due to their construction, it was difficult or impossible to drill and align holes at various angles due to the inability of the drills to remain stable. The drills would tend to swing and bounce thereby making angled drilling quite a difficult undertaking. Moreover, most machines do not allow the application of initially high down pressure (about 2000 lbs. [8896N]) on the bit. Initial pressures of this magnitude tend to cause current machines to lift and buck as a result of the reactions to the percussive hammer blows within the hole. This phenomenon makes drill string alignment difficult. The machines, although bulky, are not heavy enough to absorb the shock generated. Rather, current machines must commence drilling with a relatively low initial loading while relying on the dead weight of the increasingly longer drill string. Inasmuch as the average weight of a five foot rod is about 80 lbs/rod (36.2 kg/rod), about twenty-five rods are necessary in order to allow the hammer to operate efficiently at 2000 lbs.